1. Field of the Invention
The present invention relates to the separation and oxidation of combustible particulate residues contained in exhaust gas streams from diesel engines, as well as from other internal combustion engines, incinerators, oil or coal fired industrial or utility broilers, and other such operations.
2. Description of the Prior Art
Although this invention has broad application in many gas cleaning operations where combustible materials make up the principle particulate contaminant, by way of example, this discussion will center upon the mobile diesel engine emission application.
Increasing concern about the environment has led to tightened emissions standards for heavy duty diesel engines used in trucks and buses.
Industry has responded with a wide range of devices intended to reduce noxious fumes and particulate emissions. Efforts have concentrated on various means of removing particulates from the exhaust gas and burning the collected soot in devices called trap oxidizers.
Various forms of particulate traps have been tried including the ceramic honeycomb monolith wall flow trap.
Ceramic monolith traps consist of a honeycomb of long channels with porous walls. The open channels of the honeycomb are alternately blocked with cement at the ends so that all the exhaust flow entering the open channels must pass through the porous ceramic walls before exiting the trap.
In most cases, particulate matter is collected in a filter or trap until a substantial residue has been accumulated. All during this accumulation phase, there is a steady build up in back pressure which reduces engine output power and fuel economy. In order to keep back pressure within acceptable bounds, the trap must be periodically regenerated.
Attempts to regenerate the trap continuously on stream have proven extremely costly in terms of the energy that must be expended to constantly maintain the exhaust gas at the soot ignition temperature.
In the usual case, particulate matter is collected on a filter over some period of time varying from less than one hour to several hours. All during this time, back pressure is allowed to build up until it reaches substantial levels before regeneration is initiated. There are at least two reasons for allowing this build up to take place on the filter. The first is that the regeneration step requires energy to raise exhaust gas temperature. Therefore, by waiting longer between regenerating cycles, some energy may be saved. And second, it is desired that the filter be completely cleaned at each regeneration. By allowing the filter to fill almost completely before regeneration is initiated, it has been found that combustion can be sustained until nearly all of the accumulated soot is burned off. Thus, the buildup in back pressure during the trap fillup phase appears to be prerequisite for periodic trap regeneration according to present practice.
Regeneration is achieved by raising the collected soot temperature to above its ignition temperature and maintaining a sufficient oxygen concentration in the gas stream to oxidize the soot particles once ignition begins.
There are two known ways that ignition temperatures can be attained. The first is by engine adjustment, such as throttling the engine at cruising speed. The second is by activating some external heat source such as a gas burner or an electric resistance heater situated directly in front of the trap.
Some success has been achieved in reducing the temperature needed for combustion by adding a catalyst either to the fuel, so that it ends up on the particles themselves, or by coating the trap matrix material with catalyst directly. There have been numerous attempts to make catalyzed trap oxidizers self regenerating so that there is no need for an external heat source or engine adjustment, but to date, these efforts have not been entirely successful. A further problem is that the collected soot has a high energy content which is released when it is burned. Caution must be exercised to insure that trap matrix temperatures as well as localized thermal gradients do not exceed safe operating limits for the trap may fail due to melting or cracking.
Regardless of which method of periodic regeneration is used, storing collected soot for periodic combustion results in a number of very serious problems:
1. Increasing back pressure between regenerations results in reduced useful engine power output.
2. Special engine throttling procedures are needed to provide high temperatures for combustion, and such procedures involve high energy costs. Alternatively, auxiliary energy consuming heating means must be activated to raise the temperature of the exhaust gas whenever regeneration is needed.
3. Another very formidable problem with the periodic regeneration approach is caused by the concentrated heat energy liberated when the stored up soot is finally burned. The soot tends to burn away in a moving fire front not unlike that of a burning cigarette. As each sequential position on the filter ignites, more heat is liberated, causing intense localized hot spots in the filter. Such momentary excursions, if beyond thermal stress tolerances, can cause even ceramic traps to melt or crack, rendering them useless as filters.
4. Diesel fuel and engine oil contain small amounts of noncombustible ash, usually in the form of metallic compounds. This unburned residue remains in the trap after each regeneration cycle, slowly building up and eventually plugging the filter.
Despite intensive efforts, progress has been limited in meeting the need for a practical means of capturing and disposing of the particulate emissions from large mobile diesel engines in a reliable manner over prolonged running periods.
In summary, it is well known to sequentially collect, burn up the collected particles, and then repeat the cycle. However, this process results in a high energy penalty due to back pressure build up between regenerations, a high energy penalty in initiating regeneration, cracking and melting of the filter due to extraordinary temperature excursions brought on by intense exotherms during combustion of particles, and eventual plugging of the filter due to metallic compound residues left in the media after many regeneration cycles.
Some attempts have been made to overcome the drawbacks of the periodic regeneration approach by going to continuous regeneration of the filter. In one laboratory trial a filter was provided which remained on stream during constnat regeneration. In this case, the energy penalty was very high, the energy used to regenerate amounting to almost the same amount of energy used to power the engine. It is therefore clearly not practical to try to raise the temperature of the entire engine exhaust gas stream to the ignition temperature of the soot particles on a continuous basis.
It is also known to directly burn soot off of a filter medium. Such procedure is difficult at best. For example, not only must the temperature of the soot and surrounding gas be raised, but also, the filter segment itself must be elevated to soot combustion temperature if a complete regeneration is to be accomplished.
It is an object of this invention to provide a useful exhaust gas regenerable filter that operates in a continuous cleaning mode so that engine back pressures are maintained at consistently low levels throughout operation and do not fluctuate to extremes of high back pressure.
It is a further object of this invention to provide a regenerable exhaust gas filter that continuously burns the collected particulate matter outside the main exhaust gas exit way, in this manner making use of very low energy consuming heaters while avoiding the extremes of temperature which would otherwise result from the sudden burning of large quantities of accumulated particulate matter, as in the case of conventional prior art trap oxidizers.
It is also an object of this invention to provide a regenerable filter which is useful in bringing combustible particles to a heated surface that can be easily maintained at a regenerating temperature with low energy expense while the device is in operation.
It is also an object of this invention to provide a regenerable filter which is useful in retaining a precoating on the filter medium to thereby provide a much higher overall filtering efficiency than might be achieved with a filter that is constantly being stripped bare of built up particles.
It is also an object of this invention to provide a regenerable filter which is useful in essentially eliminating the problem of metallic compound buildup within the filter itself.
It is also an object of this invention to provide a regenerable filter which is useful in greatly enhancing the life expectancy of the filter medium by allowing it to operate essentially free from the temperature extremes brought about by combusting large quantities of builtup particles directly on the filter.